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Subject: Re: Radio static/Defroster

 human <human@nh.ultranet.com> complains:

>At least one other lister (Huw Powell) has the same problem I do:  
static between the rear defroster and the radio.<    (that would be 

>The improved reception from the new antenna only made me realize that
whenever I turned on my rear defroster I get static in the radio.

>Another data point: it is signal strength dependent.  I'll be on 
Route 4 in Durham NH listening to Maine Public Radio or WGBH in 
Boston, and turning on the defroster will trash the reception.  If I 
then switch to WUNH, whose antenna I am then right under, the static 
almost disappears.  It's not being masked by volume - even if they are 
playing something quiet this is apparent.  So, dear listers, if you 
have any knowledge of RF arcanery, what's up? 

In my 1988 90q, the radio connects to two antennas, the one embedded 
in the front window and the one utilizing the embedded rear window
defroster wiring.  This latter antenna is the only feed to the AM
circuitry.  Supposedly, for FM reception, the radio selects between 
front and back dynamically as one drives.

The rear antenna cable, the defroster grid, and the defroster power
connect at a module in the left D pillar.  This module presumably 
passes dc power to the window grid, and rf from the window to the 
radio.  It may also amplify, as it does draw power via a separate 
wire.  As this module appears to be difficult to repair, checking for 
a defective filter may be very difficult.  If this is your problem, it 
may be detected by a part swap with a known good unit.  You may have a
defective ground, either in your coax cable, or for the window grid.
Remove the right D pillar covering to check the ground.  (On my car, 
the pillar moldings pull off forward and then up.)  If there is rf
noise being injected into the defroster, it may be possible to detect 
it by hooking the radio to another antenna, and moving it into close
proximity to the rear window and trying to detect an increase in 
noise. This has to be more noise than bringing the antenna into 
proximity with the car body, because there is also an effect of 
reducing the signal, which will increase the noise after a point.

The phenomenon with WUNH vs. WGBH is due to the nature of FM signal 
processing. Ideally, FM signals are fully limited in the radio, so 
amplitude doesn't matter.  Additive noise is ignored; only zero 
crossings matter.  When the signal is weak enough, then the noise is 
additive.  If you put a 10 element Yagi antenna on your car, and 
pointed it toward WGBH's antenna, near Rts 128 and 9 in Massachusetts 
(USA), you would surely get full limiting.  Durham, NH, USA is 
probably in the fringe for car radio reception of WGBH.  For a real 
test, try WBOQ at 104.9.

Good luck

***                 ...Kirby    (Kirby A. Smith)                 ***
***              ksmith1@mailgw.sanders.lockheed.com             ***
***              [=]   kirby.a.smith@lmco.com                    ***
***  Opinions expressed herein are entirely those of the author. ***